Problem: $15$ students signed up for sports teams. $6$ signed up for soccer and the rest signed up for Quidditch. How many students signed up for Quidditch?
Explanation: We can solve this problem in two ways: $ \begin{aligned}&{6} \text{ students for soccer}\\ +& \underline{{?} \text{ students for Quidditch}}\\ =&{15} \text{ students on sports teams}\end{aligned}$ Or: $ \begin{aligned}&{15} \text{ students on sports teams}\\ -& \underline{{6} \text{ students for soccer}}\\ =&{?} \text{ students for Quidditch}\end{aligned}$ ${9}$ students